The present invention relates to polyester resin containers and more particularly to hot fill containers formed from polyethylene terephthalate.
The use of polyester resin containers for the packaging of various materials is being seen with increasing regularity. One such container is formed by blow molding polyethylene terephthalate (PET) so as to produce a biaxially oriented beverage container. It is also becoming increasingly desirable to utilize these blown containers with beverages that are packaged while at an elevated temperature to insure sterilization and/or pasteurization of the contents. These beverages are commonly referred to as "hot fill" beverages. Unfortunately, without further treatment, a PET container which has only been blow molded will not exhibit adequate thermal stability. Upon receiving the "hot fill" liquid, the container uncontrollably deforms into an unacceptable product.
Various methods have been devised to counter thermal instabilities. Two general methods have evolved. The first is mechanical and involves forming the polyester into a structural configuration which can maintain stability during hot fill. The second method broadly involves heat treating the polyester to induce molecular changes which will result in a container exhibiting thermal stability.
U.S. Pat. No. 3,733,309 discloses a PET bottle which is heat treated after molding to increase the crystallinity of the PET material and thereby reduce the thermal shrinkage involved during hot fill. U.S. Pat. No. 4,863,046 discloses performing a complex heat treatment method on a plastic preform and subsequently results in a hot fill container which exhibits a volumetric shrinkage of no greater than 1 percent.
In both of the above heat treating methods, the temperature of the PET material is significantly increased to produce the desired thermal stability and reduce deformation resulting from hot filling. However, because of the longer time interval required for heating and cooling the PET material to and from the high treatment temperatures, the cycle time for producing a PET container is increased. Another disadvantage of the above methods is that energy consumption per container produced is large and extremely inefficient.
With the above limitations in mind, it is a principle object of the present invention to produce a thermally stable blow molded hot fill container.
Another object of the invention is to reduce the cycle time required to produce a hot fill PET container.
It is an additional object of this invention to produce a hot fill PET container that utilizes reduced heat treating temperatures.
A further object of the invention is to produce a hot fill PET container which partially increases in volume during hot fill.
It is also an object of this invention to decrease the amount of energy which is consumed during the production of a hot fill container.
In achieving the above objects, the present invention seeks to offset thermal shrinkage by inducing a partial increase in volumetric size. In so doing, the present invention provides for a hot fill container having outward thermoelastically deformable panels. Furthermore, the formation of the panels, and the container itself, is done in a single molding process and does not require additional molds.
In forming the container according to the present invention, a preform or parison is blow molded into a first container configuration. The molded container is then heat treated at a relatively low temperature while it remains within the mold cavity in the first configuration. For reasons further described below, the heat treating temperature does not need to be raised to the high temperatures previously seen and required to limit shrinkage to one percent. Previously, the heat treating temperatures were commonly seen in a temperature range of 220.degree. to 260.degree. F.
After low temperature heat treating and prior to hot filling, a portion of the first configuration of the PET material is deformed inward. By deforming portions of the container inward, the resulting second configuration of the container exhibits a reduced volume. The container is then maintained in this reduced volume configuration until it has been sufficiently cooled to permit ejection from the mold.
As a result, when the container of the present invention is hot filled, the elevated temperature of the liquid causes the inwardly deformed portions of the PET container to expand outwardly, partially increasing the volume of the container, returning generally to the first blow molded configuration. The increase in volume created by the outward deformation of the PET material is sufficient to offset or counter the thermal shrinkage experienced by the PET material not deformed prior to filling.
The inwardly deformed portions of PET material expand outwardly during hot filling because the PET material in these portions has been reversely oriented. In other words, during heat treating, the stresses in the PET material are relaxed and the PET substantially "forgets" its original preform shape. The first blown configuration of the container essentially becomes a second preform shape. Upon inward deformation of portions of the container's first configuration, molecular orientation is reintroduced into the PET material resulting in the container's "memory" being that of the second preform. When hot fill liquid is now added to the container, the deformed portions "remember" the second preform shape and deflect outwardly to partially increase the volume of the container.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.